In the metal casting art, molten metal is kept in a heated crucible prior to a casting step in which the metal is ejected through an orifice. In fact, the crucible may serve as the melting vessel for the metal. Proper control of the process requires knowledge of the volume of metal contained in the crucible, as indicated by the level of the melt, for example.
One means for effecting expulsion of the molten metal from the crucible onto a moving chill surface in the casting of a ribbon of amorphous metal strip utilizes the hydrostatic head of the metal, as disclosed in Narasimhan U.S. Pat. No. 4,212,343. Accordingly, it is important to be able to determine the level of the molten metal in such a process.
Referring again to the casting of amorphous metal in thin films, Ray et al U.S. Pat. No. 4,154,283 discloses the use of an induction coil assembly to melt a metal alloy in a crucible. The melt is ejected through an orifice in the bottom of the crucible onto a chill wheel.
In such structures, it is desirable to minimize the amount of equipment necessary for measuring the level of the melt, and any such equipment at the casting machine should preferably be of the type which can be easily added to existing machinery, without requiring substantial modifications thereof.
Prior art level measuring equipment fails to meet these requirements, however.
Vischulis U.S. Pat. No. 3,519,060, for example, utilizes an additional pair of coils encircling the mold of a continuous casting machine to produce oscillations having a frequency dependent on the metal level. The reference notes that as the volume of molten metal within the encircling coils increases, the inductance of each coil decreases and the AC resistance thereof increases, due to increased eddy current losses. In addition to requiring additional coils and an oscillator, the apparatus can only be used in conjunction with a mold formed of separate, discontinuous pieces. As dislcosed in the reference, a continuous mold would prevent the induction of circulating eddy currents in the molten conductor, and it is precisely such eddy current upon which the measuring apparatus relies.
Roeske et al U.S. Pat. No. 3,326,043 requires the use of a plurality of axially aligned coils to measure the level of a conductive liquid. The self inductance of the coils is detected and compared with a reference to determine the changes therein resulting from filling of the tubular spaces within the coils by the liquid.
Hertz et al U.S. Pat. No. 3,695,107 determines the level of a conductive liquid by detecting the pulse repetition rate of a pulse transmitter which is triggered each time a pulse reflection appears at the input end of a delay line submerged in the liquid.
Playfoot et al U.S. Pat. No. 3,962,919 discloses the immersion of a bifilar wound transformer in liquid metal for level detection. The transformer is driven by a constant current power source, and the output voltage induced in the secondary winding varies with the level of the liquid metal about the coil.
Miller et al U.S. Pat. No. 3,366,873 mounts a plurality of elongated, rectangular, stacked coils on the exterior of a non-magnetic containment vessel for molten metal. The voltage induced in some of the coils by a suitably energized center coil is used to provide output signals indicative of the level of molten metal within the vessel.
Linder et al U.S. Pat. No. 4,212,342 uses a transmitter coil to induce an electromagnetic field in a bath of a continuous casting mold. A receiver coil detects the field, and changes in inductance of the coils, which is a function of the bath level, are used to indicate the metal level.
Freedman et al U.S. Pat. No. 3,456,715 discloses a number of other prior art methods for measurement of molten metal level within a continuous casting pipe or mold. For example, radiation absorption by the metal, temperature detection by thermocouples external to the mold, and coupling of acoustic energy to the interior of the container have all been used.
As is apparent from the foregoing summary, prior art measurement of liquid metal levels is typically complex, requiring the use of additional equipment, and would be difficult to apply to a crucible surrounded by an induction heating coil. Simple, straightforward measurement of a molten metal level by equipment used for the casting operation, particularly for heating, is not provided.